Method for manufacturing tire molding metal molds with a two-stage electrical discharge machining process

ABSTRACT

A method of making a mold which is utilized to manufacture a tire, the mold having a contour curved surface with broad bone portions and narrow blade portions extending therefrom, comprises a first machining stage utilizing a plurality of electrical discharge machining electrodes for approaching a workpiece used to make the mold, at different angles. Each of the electrodes used in the first machining stage are shaped to avoid the removal of excessive metal from the mold so that both the bone portions and the blade portions can extend normally from the contour curved surface. The method includes a second machining stage utilizing a further electrical discharge machining electrode which has recesses for receiving bone portions and blade portions formed by the first stage electrodes, and is movable laterally of the contour surface to form well-defined edges of the bone and blade portions.

This is a division of application Ser. No. 247,280 filed Mar. 25, 1981.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a tire manufacturing mold and amanufacturing method therefore, and more particularly to a tiremanufacturing mold and a manufacturing method, based on dischargemachining, in which a contour surface of the mold consisting of apredetermined curved surface corresponding to the outer circumferentialsurface of a tire being molded and bone portions and blade portions,both protruding on the contour surface, are integrally formed bydischarge machining.

2. Description of the Prior Art

In manufacturing a tire manufacturing mold, plate-like protrusions(hereinafter referred to as blades) corresponding to grooves as commonlyfound of the tread surface of a tire must be formed on the tire mold. Itis extremely difficult, however, to machine such blades on a tire metalmold with cutting operation, discharge machining, etc. in such a fashionthat the blades are integrally formed with the mold proper. Heretofore,therefore, the following method has been commonly used. That is,

(i) Molds of the same size and shape as the segments obtained byradially dividing a tire being molded are prepared, using gypsum, forexample, on the assumption that there exist no grooves on the tire

(ii) Metal pieces, for example, having the same cross-sectional shape asthe grooves and a predetermined height are fitted, using adhesive andother appropriate means, on the gypsum models at positions correspondingto those of the grooves on the tire. Using these gypsum models asmatrices, n pieces of their reversed molds are prepared with resin, etc.

(iii) Blades, made of stainless steel, etc., of a predetermined heightare inserted into all the grooves formed on the reversed molds by themetal pieces. The predetermined height of the blade is such that theheight of the blade excluding the portion being inserted into the grooveis equal to the height of the blade being provided on the tire mold.

(iv) Gypsum is poured into the reversed molds with the blades andallowed to cure. Thus, gypsum casting molds are obtained by removing thereversed molds. At this time, the blades inserted in the reversed moldsare moved to the casting molds with the portion thereof previouslyinserted in the reversed mold exposed on the casting mold.

(v) The gypsum casting molds thus formed are arranged in a ring shapeand used as a matrix for molding the desired tire manufacturing moldwith aluminum precision molding, for example.

With this method, the portion of the blade previously exposed on thecasting mold is embedded in the tire manufacturing mold with the portionthereof previously embedded in the gypsum casting mold exposed on thetire manufacturing mold.

Another method of providing blades on a tire manufacturing mold is asfollows.

Tire manufacturing metal molds without blade portions are firstmanufactured. And then, grooves corresponding to the cross-sectionalshape of the blades are formed on the corresponding positions of themolds by manual metalworking or discharge machining, and prefabricatedblades are embedded in these grooves.

These conventional methods, as described above, involve complexmanufacturing processes, resulting in increased manufacturing costs. Inaddition, embedded blades are very likely to become loosened, coming offin some cases.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a tire manufacturing moldmanufactured by electrodischarge machining in such a manner that acontour surface, bone portions and blade portions corresponding to theouter circumferential surface of the tire being molded are integrallyformed, and the manufacturing method thereof.

It is another object of this invention to provide a tire manufacturingmold manufactured by electrodischarge machining, which can contribute toreduction of manufacturing costs through labor saving in the manufactureof metal molds, and the manufacturing method thereof.

It is still another object of this invention to provide a tiremanufacturing mold in which blade portions are integrally formed withthe mold proper by electrodischarge machining so as to prevent the bladeportions from coming off and thereby to increase the mechanical strengthof the mold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side elevation of a tire manufacturing moldembodying this invention.

FIG. 2 is a partially enlarged cross-section of the lower half of themold shown in FIG. 1.

FIG. 3 is a developed plan view taken substantially along the line A--A'in FIG. 2.

FIGS. 4A and 4B are a front view and side view, respectively, of anexample of the electrodischarge machining equipment for use in thisinvention.

FIGS. 5A through 5C are sectional diagrams which are useful inexplaining the first-stage discharge machining in the manufacturingprocess of the tire manufacturing mold of this invention.

FIGS. 6A through 6C are sectional diagrams which are useful inexplaining the second-stage electrodischarge machining in themanufacturing process of this invention.

DETAILED DESCRIPTION OF THE INVENTION

An example of the tire manufacturing mold being manufactured by thisinvention is shown in FIGS. 1 through 3.

In the figures, reference numeral 1 refers to an upper-half mold; 2 to alower-half mold; 3 to a contour surface corresponding to the treadsurface of the tire being molded; 4 to a shoulder portion; 5 through 9to relatively broad bone portions; 10 and 11 to relatively narrow bladeportions; PL to a parting line at which the upper-half mold and thelower-half mold are matched together; and CL to a center linecorresponding to the center line of the tread surface of the tire beingmolded.

Next, an example of the electrodischarge machining equipment used forthe manufacture of the tire manufacturing mold of this invention will bedescribed, referring to FIGS. 4A and 4B.

In FIGS. 4A and 4B, numeral 13 refers to a work; 14 to a first machininghead for feeding a machining electrode in the direction shown by arrow Hin the figure; 14' to a pulse motor of the first machining head 14; 15to a spindle of the first machining head 14; 16 to a second maching headfor feeding a machining head in the direction shown by arrow h in thefigure; 16' to a pulse motor of the second machining head 16; 17 to amachining electrode mounting jig; 18 to a machining electrode; 19 to ahead support for supporting the first machining head 14; 20 to a headrotating drive unit for rotating the head support 19, together with thefirst machining head 14 in the direction shown by arrow α in the figure;21 to a carriage which is supported by a column 23 and can be lifted andlowered by a lead screw 22 in the direction shown by arrow Z in thefigure; 24 to a electrolyte tank; 25 to a machining table on which thework 13 is placed; 26 to a table rotating drive unit for rotating(servo-driving) the machining table 25 in the direction shown by arrow θin the figure by means of, for example a pulse motor or hydraulicdriveunit; 27 to a first table for moving the machining table 25, togetherwith table rotating drive unit 26, in the direction shown by arrow X inthe figure; 28 to a second table for moving the first table 27 in thedirection shown by arrow Y in the figure; and 29 to a bed, respectively.

In FIGS. 4A and 4B illustrating an example of the electrodischargemachining equipment for use in this invention, the first machining head14 drives the spindle 15 in the direction shown by arrow H in the figureby means of the pulse motor 14'. At the tip of the spindle 15 is fixedthe second machining head 16, which drives the machining electrode 18 inthe direction shown by arrow h in the figure by the pulse motor 16' viathe electrode mounting jig 17. As the operation of the second machininghead 16 will be described later, the description here is based on theassumption that the second machining head is in a still state. Themovement of the spindle 15 in the direction H by the pulse motor 14' ofthe first machining head 14 is controlled in such a manner that the gapbetween the machining electrode 18 and the work 13 can be maintainedconstant with the progress of machining in accordance with predeterminedmachining conditions such as electrode voltage, discharge current, etc.(hereinafter referred to as automatic servo-drive). In place of thepulse motors 14' and 16' for driving the first and second machiningheads 14 and 16, a hydraulic servo drive unit and other appropriateautomatic servo drive units may be used.

The electrodischarge machining equipment for use in this invention, asshown in FIGS. 4A and 4B, is capable of setting the angle of the firstmachining head 14 in the direction H, that is, the angle of themachining electrode 18 in the automatic control feed direction withrespect to the machining table 25 at a desired angle through therotation of the head support 19 in the direction α, the lifting andlowering of the carriage 21 in the direction Z, and the rotation in thedirection θ and the movement in the directions X and Y of the machiningtable 25. At the same time, the electrodischarge machining equipment isalso capable of setting the machining position of the electrode 18 fordischarge machining the work 13 at any desired position. The detaileddescription of the positioning method of the work 13 and the electrode18, which has already been proposed by the present inventors in theirU.S. Pat. No. 4,409,457 granted Oct. 11, 1983, is omitted here.

Next, this invention will be described, with particular reference to thelower-half mold 2 of the tire manufacturing mold shown in FIGS. 1through 3.

In general, a tire has on the tread surface thereof a plurality ofgrooves formed essentially vertically to the tread surface.Consequently, on a metal mold for molding such a tire, for example, thelower-half mold 2 as shown in FIG. 2, is provided with the bone portions6 and 8 and the blade portions 10 and 11 which protrude virtuallyvertically to the contour surface thereof, corresponding to theaforementioned grooves.

In discharge machining a mold such as the lower-half mold 2 shown inFIG. 2 the contour surface of which has a plurality of projectionsformed vertically to the contour surface, the feeding of a machiningelectrode (not shown) in any direction, for example, in any of thedirections shown by arrows a, b and c in FIG. 2 would result in unwantedmetal removal on any of the bone portions 6 and 8 and the blade portions10 and 11.

To overcome such unwanted metal removal, the feeding direction of themachining electrode is made changeable between the direction coincidingwith the protruding direction of the bone portion 6 and the bladeportion 10 and the direction b coinciding with the protruding directionof the bone portion 8 and the blade portion 11. Thus, the first-stagedischarge machining is performed by using different machining electrodeshaving such profiles as to prevent the aforementioned unwanted metalremoval for each discharge machining in the electrode feeding directionsa and b. The first-stage discharge machining will be described in thefollowing, referring to FIGS. 5A through 5C. In the figures, numerals 2,3, 6 through 11, 13 and 17 correspond with like numerals in FIGS. 1through 4. Numerals 18a and 18b refer to first-stage machiningelectrodes; 3a and 3b to electrode contour surfaces; 6a through 11a toelectrode recesses formed on the machining surface of the first-stagemachining electrode 18a; and 6b through 11b to electrode recesses formedon the machining surface of the first-stage machining electrode 18b,respectively.

FIG. 5A is a cross-section of the final profile of the lower-half mold 2to be manufactured in the invention.

FIG. 5B shows the first-stage machining electrode 18a used for machiningin the direction a.

FIG. 5C shows the first-stage machining electrode 18b used for machiningin the direction b. The first-stage machining electrode 18a formachining in the direction a has a profile corresponding to that of thelower-half mold shown in FIG. 5A, except for the electrode recesses 8a,9a and 11a having such profiles as to prevent unwanted metal removal, asshown in FIG. 5b. Similarly, the first-stage machining electrode 18b formachining in the direction b has a profile corresponding to that of thelower-half mold 2 except for the electrode recesses 6b and 10b havingsuch profiles as to prevent unwanted metal removal, as shown in FIG. 5C.The manufacturing method and device of the first-stage machiningelectrodes 18a and 18b have already proposed by the present inventors inthe U.S. Pat. No. 4,409,457,so the detailed description of them isomitted here.

In the first-stage discharge machining according to this invention, thework 13 is first machined into a profile as shown by a dotted line inFIG. 5B by feeding the first-stage machining electrode 18a, positionedat a location shown in the figure, in the direction a. Then, using thefirst-stage machining electrode 18b in place of the electrode 18a,discharge machining is performed in the direction b to remove theportions shown by dotted line in FIG. 5C, which are left unmachined inthe machining process shown in FIG. 5B, on the bone portions 8 and 9 andthe blade portions 11. With this process, as shown in FIG. 5C, thedesired profile of the lower-half mold 2 shown in FIG. 5A is obtained.The aforementioned first-stage discharge machining process can producethe desired profile of the lower-half mold 2 shown in FIG. 5A withoutcausing unwanted metal removal on projections since the recesses 8a, 9aand 11a of the first-stage machining electrode 18a for use in thea-direction machining and the recesses 6b and 10b of the first-stagemachining electrode 18b for use in the b-direction machining are formedinto profiles shown in FIGS. 5B and 5C.

In practice, however, it is difficult to obtain the desired profile evenwith the aforementioned discharge machining process because there aresome problems such as the wear of electrode caused with the progress ofdischarge machining and overcuts caused by secondary discharge by metalchips suspending in the discharge gap between the machining electrodeand the work, as shown in FIGS. 6A and 6B. That is, when dischargemachining (the first-stage discharge machining as described above) iscarried out by feeding the first-stage machining electrode 18a having aprofile corresponding to the desired profile as shown by a dotted linein the work 13 in FIG. 6A in the direction shown by arrow H in thefigure by means of the first machining head 14 (shown in FIG. 4A), thework 13 will be formed into a profile shown in FIG. 6B. That is, thecorners at which the contour surface 3 and each side surface of the boneportion 6 and the blade portion 10 intersect tend to be slightlyrounded. This is caused by the fact that the edges of the mouths of therecesses 6a and 10a wear out and becomes rounded since the wear of anelectrode occurs most pronouncedly at protrusions or corners on theelectrode surface, as is generally known. Furthermore, the blade portion10 tends to be tapered off toward the end thereof. This is due to theso-called overcut caused by secondary discharge by metal chips in thedischarge gap.

It is necessary, therefore, to take into account this point indetermining the profile of the recess 10a (shown in FIG. 6A) of themachining electrode 18a used for the first-stage machining. In thisinvention, the second-stage discharge machining as will be described inthe following is performed on the work 13 shown in FIG. 6B after thefirst-stage discharge machining to obtain the desired profile as shownby a dotted line in FIG. 6A.

FIG. 6C is a diagram which is useful for explaining the second-stagedischarge machining. In the figure, numeral 18c refers to a second-stagemachining electrode; 6c and 10 c to electrode recesses provided forfinish machining the bone portion 6 and the blade portion 10,respectively. The second-stage machining electrode 18c can be obtainedby machining the side walls of the recesses 10a and 6a of thefirst-stage machining electrode 18a shown in FIG. 6B to widen the sidewalls of the recesses to such an extent that the rounded parts at thecorners can be removed. The center lines of the recesses 10c and 6c thusformed are required to align with the center lines of the recesses 10aand 6 a of the first-stage machining electrode 18a. Needless to say, thesecond-stage machining electrode 18c is not limited to the one obtainedby enlarging the widths of the recesses 10a and 6a of the first-stagemachining electrode 18a, but may be a separately prepared electrode ofthe same profile.

By using the second-stage machining electrode 18c which is machined intosuch a profile to satisfy an equation (T-t=T'-t'), where T and T' arethe widths of the openings of the recesses 10c and 6c, and t and t' arethe finally finished widths of the blade portion 10 and the bone portion6 shown by dotted lines in FIGS. 6A and 6C, the blade portion 10 and thebone portion 6 can be finished simultaneously with the second-stagedischarge machining, which will be described later.

As shown in FIG. 6C, the second-stage discharge machining is performedin the following manner. The second-stage machining electrode 18c isfirst fed in the direction H by means of the first machining head 14,and then automatic servo driven by a distance (T-t)/2 in any one of thedirections h by means of the second machining head 16 (shown in FIG.4A). And then, the second-stage machining electrode 18c is automaticservo driven by a distance (T-t) in the other direction of the directionh to remove the metal left unmachined on the other side. As a result,the blade portion 10 and the bone portion 6 of the work 13 can befinished into the desired profile shown by a dotted line in FIG. 6A. Thefeeding distance of the second-stage machining electrode 18c in thedirections H and h during the second-stage discharge machining can beset to a predetermined range by means of, for example, limit switches.That is, the automatic servo driving of the machining electrode 18c inthe direction H is effected by the first machining head 14 until a limitswitch (not shown) is actuated. When the limit switch is actuated, thefeeding of the machining electrode 18c is stopped and the automaticservo driving of the electrode 18c in the direction h is started by thesecond machining head 16. If the electrode 18c and the work 13 areshortcircuited during the h-direction machining, the machining electrode18c is immediately retreated by a predetermined distance in the oppositedirections to the directions H and h. The retreat distance can also beset by presetting, for example the number of pulses fed to the machiningheads 14 and 16. Needless to say, the machining electrode 18c, onceretreated, is fed again in the direction H and then in the direction hto continue discharge machining. The retreating operation of themachining electrode 18c may be performed simultaneously in bothdirections H and h, or first in the direction h and then in thedirection H.

In the foregoing, the second-stage discharge machining involving thefeeding of the electrode in the direction h has been described. However,the blade portion 10 can be formed in a desired direction bysimultaneously performing the automatic servo driving of the machiningtable 25 (shown in FIGS. 4A and 4B) in the direction θ, or the automaticservo driving of the second machining head 16 in the direction h and theautomatic servo driving of the machining table 25 in the direction θ,and by controlling the speeds of the respective automatic servo drivingoperations in the directions h and θ at the same speed or differentspeeds, depending on the desired direction. Needless to say, theinterlocking operation of the automatic servo driving operation in thedirection H and the automatic servo driving operations in the directionsh and θ is performed in the same manner as described, referring to FIGS.6A through 6C.

As described above, this invention makes it possible to manufacture atire manufacturing mold consisting of a single block in which thecontour surface, bone portions and blade portions thereof are integrallyformed by first-stage and second-stage discharge machining operations,thus contributing to labor saving and cost reduction in the manufactureof tire manufacturing molds and making it possible to increase thestrength of the blade portions of the molds.

What is claimed is:
 1. A manufacturing method for making a tiremanufacturing mold having a contour surface consisting of apredetermined curved surface corresponding to an outer circumferentialsurface of a tire to be molded, and a plurality of bone portions andblade portions, both protruding from the contour surface and the methodutilizing an electrical discharge machining device having a machiningtable on which a workpiece is placed, a table drive unit capable ofcontrolling a rotation of the machining table, a first machining headhaving a spindle for feeding a machining electrode in a feedingdirection at a predetermined angular position, a second machining headfor feeding the machining electrode in a direction perpendicular to anaxis of the spindle, and a head support for rotatably supporting thefirst machining head, a plurality of first-stage machining electrodeseach having a portion corresponding to a common reference planecorresponding to the curved surface of the mold and recessescorresponding at least in part to at least some of the bone and bladeportions, a part of said reference plane and a part of said recessesbeing formed into such a shape so as to prevent excess metal removalduring discharge machining, in relation to the feeding direction of saidangular position, and a second-stage machining electrode having apredetermined profile for machining side walls of the bone and bladeportions; the method comprising:a first-stage discharge machiningcomprising the steps of; mounting one of the first-stage machiningelectrodes to the second machining head; setting the first machininghead at a first predetermined angular position corresponding to said oneof the first-stage machining electrodes; machining a workpiece on thetable by feeding said one of the first-stage machining electrodes in thefeeding direction of said first angular position while preventing excessmetal removal; replacing said one of the first-stage machiningelectrodes with another one of the first-stage machining electrodes; andmachining a portion of the workpiece which was left unmachined duringsaid step of preventing excess metal removal, using said other one ofthe first-stage machining electrodes; and a subsequent second-stagedischarge machining comprising the steps of; mounting the second-stagemachining electrode having the predetermined profile for machining theside walls of the bone portions and the blade portions in place of afirst-stage machining electrode on the second machining head; andmachining the side walls of the bone portions and the blade portions byfeeding the second-stage machining electrode toward the walls bycontrolling the movement of at least one of the second machining headand the machining table.
 2. A method according to claim 1, includingmachining the side walls of the bone portions and the blade portions bycontrolling movement of the second machining head to move thesecond-stage discharge electrode in a direction substantially parallelto the curved surface of the mold.
 3. A method according to claim 2,wherein the tire manufacturing mold including the bone portions and theblade portions are made of a single piece workpiece.